1. Field of the Invention
This invention relates to uplink communication protocols for use primarily with orthogonal frequency division multiple access (OFDMA) communication systems. Aspects of the invention relate to narrow band frequency division multiplexed (NBFDM) modulation protocols that operate at communication layers and sub-layers (e.g., physical and link layers) below a network protocol layer of an OFDMA communication system. Other aspects of the invention relate to noncoherent detection and enhanced methods for uplink communications in OFDMA systems.
2. Description of the Related Art
Wireless local area networks such as WiFi (e.g., IEEE 802.11) have become popular for home and office use, as well as for Internet access in public areas known as “hot spots.” More recently wireless broadband access (WBA) technologies such as WiMax (e.g., IEEE 802.16, 802.16a, 802.16e) and FLASH-OFDM™ (e.g., IEEE 802.20) have been introduced. These technologies are also known as wireless MAN (metropolitan area network) technologies. While wireless local area networks (e.g., WiFi) have a relatively short range, WBA technologies tend to have a much larger range, from a kilometer to seven kilometers, or even tens of miles or more, for example using 450 MHz FLASH-OFDM™ technology form Flarion Networks, Inc.
Wireless broadband access technology typically uses OFDM (orthogonal frequency division multiplexing) at the physical layer and uses OFDMA (orthogonal frequency division multiple access) at the layers at and directly above the physical layer to allow multiple users to share the spectral resources afforded by the multiple tones of the underlying OFDM physical layer.
Wireless local area networks only need to support a relatively small number of users in a relatively small local coverage area. Interference is usually minimized because WiFi coverage areas often do not overlap, and if they do, they can be assigned to operate in different frequency ranges or with different coding schemes or hopping sequences so that they can minimize co-network interference. However, because of the much broader coverage area of WBA base stations, WBA suffers from all of the spectral cost and interference issues normally associated with cellular communication systems. For example, much higher numbers of users need to contend for and share the same spectrum, and adjoining base stations typically provide inter-cell interference sources. In many cases, the net system capacity and operating parameters of a WBA system need to be determined by worst case conditions. In many cases these worst case conditions limit the net performance of the system.
OFDM transmission assumes that an estimate of the phase of a sub-band channel between a mobile unit the base station is available. In the downlink, the channel phase is estimated by sending known information, i.e., phase-reference symbols, on one or more specified pilot channels during each OFDM symbol interval. Because these phase-reference symbols are broadcast to and are used by all the mobiles in a given base-station antenna's coverage area, only a small amount of spectrum is needed to support pilot signaling. However, in the uplink of an OFDMA system, a similar scheme would require each mobile to transmit separate phase reference symbols back to the base station. Given N mobiles in a given antenna's multiple access coverage area, this would require N times as many sub-band channels to be used to support phase-reference pilot signaling as was required to achieve the same effect in the downlink. Also, because the downlink signal is transmitted by a single base station, all the different tones are perfectly synchronized to the pilot tone(s) and to each other. This allows the OFDM receivers in subscriber stations to readily demodulate the downlink signal. However, imperfect synchronization and different path delays and Doppler effects create a loss in orthogonality in the uplink. Hence in the uplink, the OFDMA performance is further reduced due to intercarrier interference effects due to imperfect synchronization among multiple simultaneously transmitting mobile units and an attendant loss of orthogonality within the composite uplink signal as observed at the base station.
Many proposals have been made to deal with the difficulties in the uplink of OFDMA systems. One category of proposal is to use an altogether different protocol in the OFDMA uplink. That is, it has been proposed to use OFDM in the downlink of an OFDMA system, and to use something else, such as a third generation (3G) cellular-technology in the uplink, for example the HDR™ wireless data air interface available from Qualcomm, Inc.
Another class of proposals has been to try to develop efficient phase estimators to deal with the unavoidable synchronization and orthogonality errors inherent in the uplink (see the patents cited in the accompanying information disclosure statement for further details). These types of proposals seek to improve the situation via signal processing techniques similar to the idea of adaptive equalizers, but on a per-mobile/per channel basis. While these techniques can provide improvement, they are still limited in their ability to correct all of the underlying problems that are raised in an OFDMA uplink where different subscriber-side mobile units are not perfectly synchronized and give rise to different and time-varying multi-path delay effects and Doppler effects.
Given the inherent difficulties in the prior art OFDMA uplink protocols, it is contemplated by the present invention that uplink technologies are needed that can effectively and inherently overcome the synchronization and other related channel mismatch problems that plague prior art OFDMA uplink performance.